The invention relates to an ultrasonic probe.
Ultrasonic probes are utilized, for example, in medical sonography.
So-called ultrasonic pulse echo processes are known wherein ultrasonic pulses are beamed into tissue and the signals reflected by the tissue are recorded, for example in order to draw conclusions regarding the depth structure of the tissue from the time curve of the reflected signals. In the so-called ultrasonic Doppler methods, the shift in the frequency of the reflected signal with respect to the beamed-in signal is measured to determine therefrom, for example, the velocity of the reflecting structure. In most cases, ultrasonic pulses are used having a carrier frequency f.sub.o, a pulse duration T.sub.p, and a repetition rate f.sub.w (pulses per second).
In order to obtain a high local resolution in these methods, it is necessary to provide a short sound wavelength .lambda..sub.o and/or a high carrier frequency f.sub.o (.lambda..sub.o =c.sub.o /f.sub.o ; .lambda..sub.o : average sound wavelength, c.sub.o : average sonic velocity of the medium for the energized wave type). On the other hand, the carrier frequency f.sub.o must not be too high because otherwise the attenuation of the sound waves by absorption or scattering becomes too extensive. For many media, in the relevant frequency ranges, the ultrasonic absorption increases in an approximation with the second power of the frequency and scattering increases even with the fourth power of the frequency. Therefore, in medical sonography, one is generally restricted to the frequency range of a few MHz. However, for examinations wherein only a minor penetration depth is necessary, the frequency can amount under certain circumstances to up to 20 MHz, for example, G. S. Werner et al., "Intravaskulare Ultraschalldiagnostik" [Intravascular Ultrasonic Diagnostics], Dtsch. med. Wschr. 115: 1259 (1990).
In the pulse echo processes, it is advantageous to utilize pulses of a short duration T.sub.p because, for example, the depth of focus of the probe is also limited by the spatial pulse length L.sub.p .apprxeq.c.sub.o .multidot.T.sub.p. On the other hand, it makes no sense to use pulses having a duration T.sub.p &lt;1/f.sub.o because in such a case there is no longer any gain in spatial resolution.
The Doppler methods usually require a minor local resolution but, on the other hand, it is important to measure velocities precisely. As a rule of thumb, it can be noted that the measuring accuracy is on a scale of .DELTA.c.apprxeq.c.sub.o .multidot..DELTA.f/f.sub.o wherein the frequency width .DELTA.f of the sonic pulses is reciprocal to the pulse duration, .DELTA.f.apprxeq.1/T.sub.p and .DELTA.c.apprxeq.c.sub.o /(T.sub.p .multidot.f.sub.o): thus, high frequencies and long pulses can be advantageous. In the extreme case, continuous-dash signals are even employed with T.sub.p &gt;&gt;1/f.sub.o ; local resolution in this case is only produced by the directional effect of the sonic head.
The sonic head of an ultrasonic probe normally consists of an imaging system, the so-called ultrasonic directional element, and electromechanic ultrasonic transducers directly mounted thereto. In the simplest instance, a sonic head comprises a substantially homogeneous directional element, the typical transverse dimensions of which, characterized by D, are large as compared to the average sonic wavelength in the tissue to be examined: D&gt;.lambda..sub.o. In this case, the directional element exhibits a directional characteristic, i.e. the emitted ultrasonic energy is distributed over a narrow zone about its axis, and the element receives essentially only sound emanating from a narrow zone about its axis. If the directional element is, for example, a rotating lobe radiator, then the full aperture angle ("3 dB width") for the "main lobe" amounts, in the arc measure, to .DELTA..theta..apprxeq.0.52.multidot.(.lambda..sub.o /D). A description of the rotating lobe radiator is found, for example, in H. Kuttruff, "Physic und Technik des Ultraschalls" [Physcis and Technology of Ultrasound], S. Hirzel publishers, 1988 (ISBN: 3-7776-0427-5). The manner in which the directional characteristic of ultrasonic directional elements, such as the rotating lobe radiator, can be improved is known; this can be done, for example, by the use of focusing elements. In this way, regions for ultrasonic examination can be highlighted along the axis of the directional element (regions of increased ultrasound intensity during beaming and/or reception), in order to be able to investigate these regions, for example, by means of Doppler methods with long pulses.
The examination of spatial structures requires scanning in one or two directions. In the simplest case, a single sonic head is guided, for this purpose, along the surface of the body; the measuring results thereof are combined into a two-dimensional image (path and depth). An overview regarding scanning methods is found in R. Millner (editor), "Ultraschalltechnik-Grundlagen and Anwendungen" [Ultrasonic Technique--Fundamentals and Applications], Physik publishers Weinheim, 1987 (ISBN: 3-87664-106-3).
Percutaneous studies of deep structures have the drawback, inter alia, that the ultrasonic waves are too strongly attenuated at the site of the investigation, whereby local resolution is diminished. Furthermore, ultrasonic waves can penetrate only poorly through bone or lung tissue. In this connection, it is known to mount ultrasonic directional elements and transducers at the distal end of probe tubes. These usually flexible probe tubes or catheters can be introduced into the patient and the examination can be performed intracorporeally.
In medicine, such ultrasonic probes are combined with a large number of treatment devices. U.S. Pat. No. 4,887,605 describes, for example, a catheter for laser angioplasty wherein an optical fiber is arranged for the transmission of laser light. The sonic head (directional element with transducer) at the distal end makes it possible to control a laser ablation process.
Combination of sonic probes with endoscopes (endosonographs) is especially advantageous. The sonic probe can supplement the endoscope, as known from European Patent 00 46 987. In this reference, the sonograph serves for determining the distance of the aimed-at object from the observation window so that its size can be accurately determined. The sonograph can also be an independent diagnostic device of the endosonograph. By means of an introduced sonic head, it is possible, for example, to examine the opaque tissue surrounding the probe head while, with the use of the endoscope, the probe head is precisely positioned in the patient. In most cases, the endoscopes are operated in linear sight and the sonographs in lateral sight; however, there are many other designs. For example, in European Patent 00 61 332, an endosonograph is disclosed wherein an additional peripheral window for optical lateral viewing is located at the probe head.
In order to attain local resolution with the ultrasonic probes, several processes can be considered. DOS 3,910,336, for example, describes a sector scanning method: The cylindrical probe head has a window in the peripheral direction, for radial ultrasonic beams, permitting an azimuthal sector scan about approximately 360.degree.. However, for this purpose electrical drive means are required in the probe head whereby the probe head becomes relatively cumbersome; such probes can be used for examinations of the gastrointestinal tract. The probe can also be rotatable from the outside by way of a hollow shaft, for example, as known from DOS 3,816,982.
The conventional sonic heads (directional elements with transducers) of the ultrasonic probes exhibit considerable disadvantages, for example for intracorporeal applications:
Ultrasonic transducers are temperature-sensitive. Thereby, problems are encountered with regard to sterilizability. PA1 Ultrasonic transducers (frequently piezoceramic elements) develop heat within the probe head; this heat is difficult to remove due to the compact structure. PA1 The probes are relatively thick; for this reason, their utilization, for example, in arthroscopy and bronchoscopy is considerably restricted. PA1 The operation of the transducers requires relatively high electrical voltages. Therefore, the transducers must be well insulated. If it is necessary for the probe heads to be of the type that can be opened, for example for cleaning purposes, then there are cracks through which body fluid can enter and cause short-circuiting. PA1 D.multidot.f.sub.o .ltoreq.7000 m/s if the sonic waveguide consists of materials with a Poisson ratio .upsilon. smaller than 0.20, and PA1 D.multidot.f.sub.o .ltoreq.4200 m/s in case of other materials, if axial waves are transmitted and PA1 D.multidot.f.sub.o .ltoreq.14,000 m/s for SWG based on Be, PA1 D.multidot.f.sub.o .ltoreq.6100 m/s for SWG based on silica glass, and PA1 D.multidot.f.sub.o .ltoreq.5100 m/s for SWG of other materials, if torsional waves are to be transmitted.
In order to solve these problems, it is suggested, for example, in DOS 3,219,118 to mount special electrical contacts at the probe head, making it possible to effect a simpler mechanical exchange of components. DOS 3,537,904 proposes to decouple the ultrasonic transducer in the probe head from the control and display units. This decoupling is to take place inductively, capacitively, or via optical couplers. These devices, however, have not overcome the deficiencies in catheter-guided sonography.
U.S. Pat. Nos. 4,407,294, 4,428,379 and 4,431,006 describe, for example, lancets designed as hollow needles which are inserted by puncturing through the skin, sound conductors, preferably of steel, being introduced into such lancets and pushed through to the tip in order to examine the tissue at the tip. It is difficult with the use of this method to obtain ultrasonic images--for investigating expansive structures, the lancet would have to be newly applied for each examination point. Furthermore, a problem resides in acoustically decoupling the lancet and the sonic waveguide--due to sonic overcoupling, the signals are falsified, for example. For this reason, sound-absorbing layers are arranged in the hollow needle; thereby, the needle becomes thick, and its usefulness is restricted. Furthermore, the sound conductor is too rigid and too short to be utilized in catheters.
WO PA 87 01 269 describes a process for the transmission of ultrasonic images wherein sound waves are transmitted with fibers and/or fiber bundles from a transducer outside of the body to an organ inside the body, and the reflected waves are again transmitted back. Under the prerequisite that the fiber and/or the fiber bundle contacts the organ to be examined, this process is suitable, at best, for investigating the thin layer of tissue directly adjoining the fiber end and/or the fiber bundle end. The examination of depth structures, however, is impossible. Images with areal expansion could be obtained only by the use of fiber bundles wherein the area that can be imaged is limited by the cross-sectional area of the fiber bundle. One disadvantage in the fiber bundle design resides in that the sound wavelength, in the indicated frequency range of less than 10 MHz, is still so large that a strong over-coupling of sonic energy takes place between the fibers of the bundle, destroying the image information. It has furthermore been found that a very strong damping and dispersion of sonic energy occur in fiber bundles, due to the relative movement of the fibers.
The invention is based on the object of providing an ultrasonic probe with high local resolution and great depth focus, which is electrically safe, exhibits only minor evolution of heat in the probe head, and permits a compact structure. The ultrasonic probe is to be suitable, in particular, for use in medical sonography.